Embodiments of the present invention pertain generally to implantable medical devices, and more particularly to implantable medical devices that track ST segment shifts utilizing histograms.
An implantable medical device is implanted in a patient to monitor, among other things, electrical activity of a heart and to deliver appropriate electrical and/or drug therapy, as required. Implantable medical devices (“IMDs”) include, for example, pacemakers, cardioverters, defibrillators, implantable cardioverter defibrillators (“ICD”), and the like. The electrical therapy produced by an IMD may include, for example, pacing pulses, cardioverting pulses, and/or defibrillator pulses to reverse arrhythmias (e.g., tachycardias and bradycardias) or to stimulate the contraction of cardiac tissue (e.g., cardiac pacing) to return the heart to a normal sinus rhythm.
Cardiac ischemia is a condition whereby heart tissue does not receive adequate amounts of oxygen and is usually caused by a blockage of an artery leading to heart tissue. Ischemia arises during angina, acute myocardial infarction, coronary angioplasty, and any other condition that compromises blood flow to a region of tissue. When blockage of an artery is sufficiently severe, the cardiac ischemia becomes an acute myocardial infarction (AMI) also referred to as a myocardial infarction (MI) or a heart attack.
Many patients at risk of cardiac ischemia have pacemakers, ICDs or other medical devices implanted therein. Electrocardiograms (ECG) acquired from these devices are useful for diagnosing ischemia and locating damaged areas within the heart. ECGs are composed of various waves and segments that represent the heart depolarizing and repolarizing. An ST segment (STS) represents the portion of the cardiac signal between ventricular depolarization and ventricular repolarization.
Techniques have been developed for detecting cardiac ischemia using implanted medical devices. Some conventional IEGM-based ischemia detection techniques seek to detect ischemia by identifying changes in the elevation of the STS from the baseline of the IEGM that occur during cardiac ischemia. Elevation of the STS in an IEGM (e.g., an acute voltage shift in the STS of an IEGM) may result when there are abnormalities in the polarizations of cardiac tissue during an acute myocardial infraction (MI). Deviation of the STS from a baseline is a result of injury to cardiac muscle, changes in the synchronization of ventricular muscle depolarization, drug or electrolyte influences, or the like.
STS variations or deviations, for example, voltage shifts in the STS of an intracardiac electrogram in an ICD or pacemaker can be recorded and stored in different formats for analysis. For example, STS variations can be stored as STS histograms that may be used to assess and evaluate the existence, exacerbation and alleviation of myocardial ischemia. The STS variation histograms may include a plurality of different bins representing events or values corresponding to the STS variation data. The bins store data that can be used to track the occurrence of the events or data. For example, event or data counts may be stored in each of the bins corresponding to a particular event or data value (or range of data values). The bins may correspond, for example, to incrementally changing variances of the mean of the STSs.
However, conventional implantable devices do not allow tracking of a patient's STS variations for heart rate ranges over different periods of times, for example, using STS histograms. These conventional implantable devices typically store raw STS and other cardiac data. The amount of collected raw data increases rapidly. However, the storage capacity of these implantable medical devices may be limited and not allow much raw data to be collected over a certain time period. For example, the memory within these implantable devices may not be able to store the total amount of collected raw data over a period of time (e.g., a period of interest), thereby resulting in some of the data being deleted. Accordingly, conventional devices may not provide sufficient information over a sufficient time period of time to allow for proper analysis of STS information to determine ischemic conditions and the like. Thus, a need remains for tracking in implantable medical devices STS variations for heart rate ranges over different periods of time using STS histograms.